1. Field of the Invention
The present invention relates to a wireless communication system, a wireless communication device, and a valid path detection method used for the system, and more specifically to a wireless communication system including a valid path detecting function.
2. Description of the Related Art
In CDMA (code division multiple access) wireless communications, techniques of a path search, a RAKE combining, and an SIR (signal-to-interference power ratio) estimation are generally known.
In the CDMA wireless communications, a sequence (pilot signal) predetermined between a transmitter and a receiver is first inserted on the transmitter, and a convolution sum between a received signal and a pilot sequence is obtained on the receiver, thereby obtaining correlation power (delay profile) for a predetermined time period. FIG. 11 shows an example of the delay profile.
The path search technique is to detect a sample exceeding a predetermined path power level threshold from the delay profile. In FIG. 11, a path #1, a path #2, and a path #3 exceed the threshold, and a searcher detects the paths as path timing.
Then, based on the path timing detected by the searcher, the received signal is despread, the RAKE combining for the despread signal is performed, and demodulation, decode, CRC (cyclic redundancy check), etc. are performed. HARQ (hybrid automatic repeat request) is known as a technique of requesting retransmission of data to a transmitter when there is a CRC error.
FIG. 12 shows the operation of the HARQ at a receiver. In FIG. 12, the CRC issues permission after two retransmitting operations. In HARQ, since the likelihoods from demodulated signals can be added, a time diversity effect can be obtained.
That is, in FIG. 12, the processes are performed on new data in the HARQ operations of the receiver in the order of “path detection”→“despread”→“demodulation”→“calculation of likelihood #1”→“decode”→“error detection”→“issue of data retransmission instruction”. On the first retransmitted data by the data retransmission instruction, the processes are performed in the order of “path detection”→“despread”→“demodulation”→“calculation of likelihood #2”→“combination of likelihood #1+likelihood #2”→“decode”→“error detection”→“issue of data retransmission instruction”.
On the data in the second retransmission by the data retransmission instruction, the processes are performed in the order of “path detection”→“despread”→“demodulation”→“calculation of likelihood #3”→“combination of likelihood #1+likelihood #2+likelihood #3”→“decode”→“error detection”→“issue of new data transmission instruction”.
Furthermore, to maintain the synchronization of reception timing between mobile terminals at a base station, there is an adaptive transmission timing control method (for example, refer to RCS2003-141, the Institute of Electronics, Information and Communication Engineers Technical Report). In the multiple access system by DS-CDMA (direct spread-CDMA), by securing the synchronization of reception timing between the terminals at the base station, the orthogonalization between reference paths can be secured between the terminals simultaneously accessing the base station, thereby improving the frequency use efficiency.
FIG. 13 shows a method of obtaining the delay profile of each of mobile terminals MS1 and MS1 at a base station. In the adaptive transmission timing control, based on the maximum propagation delay time (Tmax) of the incoming path at the base station, the propagation delay time (Tmax−T1) and (Tmax−T2) of the reference paths (valid path of highest power or leading valid path) T1 and T2 of the terminals MS1 and MS2 are calculated. The propagation delay time (Tmax−T1) is added to the control information for the terminal MS1, the propagation delay time (Tmax−T2) is added to the control information for the terminal MS2, and therefore each of the terminals MS1 and MS2 is notified of a transmission timing offset value.
Each of the terminals MS1 and MS2 demodulates the control information, adds the transmission timing offset value notified from the base station to a predetermined initial value, and offsets the transmission timing by the result to transmit data. Thus, the synchronization of the reference path timing between the terminals MS1 and MS2 is established at the Tmax position in the base station as shown in FIG. 14.
In the wireless communication system (for example, refer to “Physical Channels and Multiplexing in Evolved UTRA Uplink” (3GPP TSG RAN WG1 #42 on LTE R1-050850)) in the uplink proposed by the 3GPP LTE (3rd generation partnership project long term evolution), the cyclic prefix (CP) is inserted into a transmission signal, and the orthogonalization among users in the FDMA (frequency division multiple access) in the uplink is secured when the propagation delay difference between the reference paths of the users is equal to or less than the CP length.
In this system, there is a proposition of periodically transmitting a transmission timing measuring signal from a terminal to a base station using an uplink synchronization channel to realize the adaptive transmission timing control. In addition, in this system, there is indicated the possibility to replace the uplink synchronization channel with a pilot channel.
Furthermore, in estimating the signal-to-noise+interference power ratio, there is a method of estimating it from a channel estimation value or the like based on the path timing result (for example, refer to “W-CDMA Mobile Communication System, Chapter 2, Wireless Transmission System, 2-2, W-CDMA Basic Transmission Technology, c. High-Speed TPC based on the SIR Measurement” (edited by Keiji Tachikawa, published by Maruzen Publication, issued on Jun. 25, 2001, pp 53-55)). The chip noise power for use in a recently noted frequency domain equalizer is obtained by estimating signal power from a channel estimation value after obtaining the channel estimation value based on a path timing result, and subtracting the estimated signal power from the send/receive power. Since the dispersion of the noise power is large, a time average value is obtained using an oblivion coefficient.
The signal-to-noise+interference power ratio requires a high-accuracy estimation for use as a parameter of adaptive modulation/demodulation technology. If the chip noise power estimation accuracy is high, it is considered that the performance of a frequency domain equalizer is enhanced.
In the CDMA wireless communications, it is important to enhance the detection ratio and accuracy of the foremost path. Especially, in the path search, there can be the case where no path exceeding a path level threshold can be detected in a delay profile as shown in FIG. 15.